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Subject: Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     jirislaby@kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-kernel@vger.kernel.org,
        syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
References: <20210807190513.3810821-1-phind.uet@gmail.com>
 <YRYgSZwivcPPMhrS@kroah.com> <7323d825-f596-223b-a063-7539a6a57107@gmail.com>
 <YR0TRf+283ujBxbv@kroah.com>
From:   Phi Nguyen <phind.uet@gmail.com>
Message-ID: <dd30778d-2b12-f784-d00e-3b977358e5da@gmail.com>
Date:   Mon, 23 Aug 2021 00:09:05 +0800
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On 8/18/2021 10:03 PM, Greg KH wrote:
> On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:
>> On 8/13/2021 3:33 PM, Greg KH wrote:
>>>>
>>>> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
>>>> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
>>>> ---
>>>>    drivers/tty/tty_io.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>>>> index e8532006e960..746fe13a2054 100644
>>>> --- a/drivers/tty/tty_io.c
>>>> +++ b/drivers/tty/tty_io.c
>>>> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>>>>    	ld = tty_ldisc_ref_wait(tty);
>>>>    	if (!ld)
>>>>    		return -EIO;
>>>> +	tty_buffer_lock_exclusive(tty->port);
>>>>    	if (ld->ops->receive_buf)
>>>>    		ld->ops->receive_buf(tty, &ch, &mbz, 1);
>>>> +	tty_buffer_unlock_exclusive(tty->port);
>>>
>>> Did this fix the syzbot reported issue?
>>>
>>> thanks,
>>>
>>> greg k-h
>>> Yes, this fixed the syzbot reported issue.
>>
>> The lock is grabbed in flush_to_ldisc() and paste_selection().
>> Actually, I follow the document in tty_buffer.c, where it say the callers to
>> receive_buff() other than flush_to_ldisc() need to exclude the kworker from
>> concurrent use of the line discipline.
>>
>> And function tiocsti() has the following comment:
>> /* FIXME: may race normal receive processing */
>> that why I add lock in this function.
> 
> As you are fixing this FIXME, please remove it in this patch, otherwise
> we will not know it is resolved :)
> 
> Can you add that to the change and submit a new version?
> 
Yes, I will submit a new version.

> Also, this should go to stable kernels, right?  Any idea how far back?
>
I have no idea about this question, but I see it meets specified 
requirements in stable-kernel-rules.rst

BR,
Phi.